Multi-layered sound absorbing panel



April 4, 1967v vlAN-NIAN CHEN ETAL 3,312,304

MULTI-LAYERED SOUND ABSORBING PANEL Filed Nov. 2o, 1964 2 sheets-sheet 1 Ply. 1

Inventar;

YIAN NIAN CHEN ERICH PHILIPP April 4, 1967 YIAN-NIAN CHEN ETAL 3,312,304

MULTI-LAYERED SOUN-D ABSORBING PANEL Filed Nov. 20, 1964 2 Sheets-Sheet 2 Fig. 4

Jm/e'fr YIAN NIAN CHEN ERICH PHILIPP BY fz 2 mim JQWWMJA-w/ ATTORNEYS yUnited States Patent @ffice 3,312,304 p Patented Apr. 4, 1967 3,312,304 MULTI-LAYERED SOUND ABSORBING PANEL Yian-nian Chen and Erich Philipp, Winterthur, Switzerland, assignors to Sulzer Brothers Limited, Winterthur, Switzerland, a Swiss company Filed Nov. 20, 1964, Ser. No. 416,676 Claims priority, application Switzerland, Nov. 21, 1963,

' 14,304/ 63 4 Claims. (Cl. 181-33) The present invention pertains to a construction for sound absorbent `material which can be used on space partitions, for example the walls or ceilings in rooms containing weaving machinery.

Minerahwools such as glass wool and rock wool are among the materials with which good sound absorption may be obtained. In general therefore the ceilings, and under certain circumstances the side walls of the space in which the sound is to be damped are fitted with stiff foraminous plates or sheets behind which mineral wool is disposed. In order to prevent the fall of dust and of particles of the fibrous mineral wool material down into the room, it is customary to envelop the mineral wool in air-pervious acoustic palper behind the foraminous plates.

In chemical or textile plants, for example in weaving rooms, there is often encountered a high humidity which penetrates the acoustic paper and which is absorbed by the mineral wool. For such spaces therefore envelopment of the sound absorbent material in paper is unsuitable, and hence the paper has in such cases been replaced by synthetic films such as polyethylene. Measurements have shown however that in such prior art construction the enclosure of the mineral wool in a synthetic film envelope adversely affects the sound absorbent properties of 'the material. Thus, while good sound absorption is achieved with constructions employing a-foraminous plate and mine-ral wool without enveloping film and also with constructions employing mineral wool enveloped in a film but without the foraminous plate, measurements have shown that the sound absorption declines sharply at audio frequencies from one kilocycle (1 kc.) upwards, when the construction employs mineral wool enclosed in a synthetic film and a foraminous plate in front thereof.

That is to say, provision according to the prior art of a synthetic film enveloping the mineral wool as a protec- -tion against dampness has reduced the efficacy of the resultant sound absorbent material unless the lforaminous plate, desirable for -reasons of structural rigidity and permanence, is dispensed with. p

The invention provides a construction employing mineral wool, a synthetic film enveloping it, and also a foraminous plate (and hence possessing the resistance to moisture provided by the film and the structural strength provided by the plate), whi-ch possesses at least over the range from 1 to 6 kc. sound absorption properties as good as those heretofore available with the combination of a foraminous plate and mineral wool without the woolenveloping film or with the combination of mineral wool in an enveloping film but without the structurally desirable foraminous plate in front of it. The sound absorbent material of the invention comprises a layer of mineral wool which is enclosed in a non-porous synthetic film for protection against humidity, and a foraminous plate on the side of the mineral wool facing the room from which sound is t-o be absorbed, and there is provided between the foil 'and the perforated plate a space amounting to at least half the diameter of the holes in the plate. The sound absorbent material of the invention may be made up in panels of any. convenient size.

The spacing is preferably advantageously provided by ture throughout the several figures.

means of a foam-like material such as polyurethane for example having open pores, or by means of a grid-like intermediate structure maintaining the spacing. The expression foam-like material with open pores is to be understood as meaning a foam-like material, for example a synthetic foam, or foam rubber, whose pores extend as small channels through the layer of foam, and which foam-like material is hence pervious toair. A felt-like material may also be used` as the spacing element if the humidity is not too high in the space wherein sound absorption is to be achieved.

The invention will now be further described in terms of a number of non-limitative examples with reference to the accompanying drawings in which:

FIG. l is `a fragmentary transverse section through a sound-absorbent construction according to the invention, wherein however for punposes of generality the space between the sound-absorbent material and the foraminous plate has been left empty;

FIGS. 2 and 3 are sectional views similar to that of FIG. 1 showing two embodiments of the invention; and

FIG. 4 is a diagram of sound absorption as a function of sound-Wave frequency.

Like reference characters denote similar parts of struc- Referring to FIG. l, the perforated or foraminous plate 1 has the form of a thin, flat sheet. It may be provided with perpendicular sides, as shown at 1 in FIGS. 1 and 2, to form a shallow open box. It may be made of customary building materials such as sheet metal, gypsum, reconstituted wood chips or synthetic material or the like. The sheet l is provided with a multiplicity of holes 9, with either regular or virregular spacing between them. These holes may have a diameter of up to a few millimeters. The fraction of the surface area of the plate to be occupied by holes may be 15-30%. A sound absorbent material such as mineral wool indicated at 2 is disposed behind this plate, this material being enveloped by means of a synthetic plastic film 3 as .a protection against humidity. The thickness of the film may amount for example to a few hundredths of a millimeter whereas the thickness of the mineral wool layer 2 may be some l0 to 50 millimeters. In accordance with the invention there is maintained a space between the foil 3 and the perforated plate 1 which amounts to at least half the diameter of the holes in the plate 1.

In FIG. l this spacing is indicated as a simple air space 7, which however is shown in FIG. 2 as filled with an open pore foam material 5 and in FIG. 3 with a grid-like member 6 which may be made up of corrugated elements likewise made of synthetic material. In FIG. 3, in contrast to FIGS. l and 2, the plate 1 is shown simply as a flat sheet, without side walls such as are indicated at 1' in FIGS. l and 2.

The spacer element 5 of open pore foam material (FIG. 2) effects a supplementary lsound absorption by reason of the fact that the sound waves arriving thereat penetrate into a multitude of channels of small crosssection extending in different directions into Athe material 5. The sound waves are thus split and fragmented, and are uniformly disposed over the surface, so that the energy of the sound waves is somewhat reduced in these channels before the waves reach the film 3 and the mineral wool 2.

Reference character 8 indicates the ceiling or wall of a room provided with sound absorbent panels according to the invention.

The panels, according to the invention may be made of any desired shape or dimensons, for example square asiasoal 3 posed behind the plate 1 plural-packings each comprising a layer of mineral wool 2 enclosed in a film 3. As shown in FIG. 2, plural film envelopes may be provided behind a single plate 1.

The improved sound absorbent operation of the panels according to the invention having a spacing between the perfor-ated plate 1 and film 3 may be explained as follows:

The incident sound waves produce minute air pistons in the holes 9 of the forminous plate 1, as shown in idealized form at reference character 4 in FIG. 1. These air pistons operate as point sound sources for the sound propagating into the plate. In the construction of the prior art in which the film 3 lies directly against the plate and hence directly at the holes 9, theformation and oscillation of the air pistons 4 is largely prevented. In consequence in such prior art constructions the lilm 3 is brought into local oscillation in the vicinity of the individual holes. By reason of the small compliance of the film to point excitation, there results a relatively high reflection of the incident sound waves.

By means of the invention there is permitted the formation and unhindered oscillation of the air pistons 4. In consequence, the sound waves in air which form at the individual holes 9 and which propagate out therefrom behind the plate 1 as indicated at 10 in FIG. 1 c-an combine by superposition into a wave front. In this way the sound arrives at the film 3 in the form of a wave front and does not generate locally limited oscillationbut rather oscillations of large portions of the lilm surface. The film is able to follow these oscillations produced bythe wave fronts much more readily than the excitation of local or -punctiform oscillations existing in prior art structures not including the spacing 7. In this way, reflection of sound Iat the holes 9 and film 3 is great- ,Y ly diminished so that the sound absorbent effect of the whole construction is substantially increased.

The open pore foam material 5 (FIG. 2) improves the sound absorption ,by reason of t-he fact that the sound waves spreading out therein from the holes 9 are divided into a large number of small channels. This also promotes the excitation of the film 3 to oscillation in large areas rather than locally.

In FIG. 4 the Scale of ordinates represents the absorption coefficient a obtained in a Kundts tube, with unity on this scale representing 100% absorption, 0.3 representing absorption, and so on. The scale of abscissae represents frequencies in cycles per second, logarithmically presented.

The measured results for a .as shown in FIG. 4 were measured at a mineral wool layer 2 -of about 15 millimeters thickness. The curve a shows the values of for the mineral wool alone, i.e. without enveloping lm and without the perforated plate 1, whereas the curve B shows the values for the same mineral wool layer enveloped in a polyethylene lm some microns thick,

again without the plate 1.

Measured values obtained with a perforated plate 1 in which the holes 9 occupy some 20% of the area and with which there was used a mineral wool 2 without a iilm, give a curve nearly coincident with the curve b. Consequently there has been given no curve for the case of mineral wo-ol without a film 3 but with a perforated plate 1. For the combination of a perforated plate 1 and the sam-e mineral wool layer 2 enveloped in lm (but without the spacing of the invention) the measured values were widely distributed, falling within the area indicated with diagonal shading and identified at c in FIG. 4. The basio for this wide distribution of values lies in the fact that the position of the lilm 3 with respect to the perforated plate varied from measurement to measurement so that in these various measurements the air pistons v4 could to some extent be formed and thus affect the absorption coeicient a.

Lastly the curve d shows the operationof the sound absorbent material of the invention in which the spacing between the foil 3 and the plate 1 was-lled with a foam material 5 (FIG. 2) having open pores and having a 5 thickness of about 3 millimeters. The wool layer, lilm and plate of curve d were the same as the corresponding elements in the construction whose absorption is represented by the area c. It will be observed that the curve d at frequencies above about 1700 cycles per second 10 above the diagonally shaded area c, indicating forl curve d better sound absorption than was obtained with the combination of mineral wool, enveloping film and foraminous plate but without spacing of the plate from the film.

Whereas the size of the holes 9 in the plate 1 has no substantial influence on the curves for a, the location in ,frequency of the maximum for the :absorption coefficient a may be shifted by changing the fraction of the plate surface occupied by the holes.

While the invention has been described herein in terms of a number of preferred embodiments, the invention itself is not limited thereto; rather the invention comprehends various -modications in and departures from the constructions shown in these examples which may be made within the spirit `and scope of the invention las set forth in the appended claims. For example, the holes in the plate or sheet 1 need not be all of the same size; in such a case, the spacing between the foraminous plate 1 and the sound absorbent material7 s-uch as mineral wool enveloped in ra plastic film, should be at least one-half the diameter of the greatest iholes in the plate.

We claim:

1. A sound absorbent construction material comprising a foraminous sheet, a layer of fibrous sound absorbent material disposed `adjacent the sheet and substantially parallel thereto, a synthetic film enveloping said layer,v

and means to space said layer from the sheet by a distance at least equal to one-half the diameter of the .holes in the sheet.

40 2. A sound absorbent structure comprising a sheet having multiple holes therein, a layer of mineral wool disposed adjacent the sheet and extending substantially parallel thereto, a non-porous synthetic plastic lm enclosing said layer, and means to space said layer from said plate by a distance amounting to 4at least half the average diameter of the holes in said sheet.

3. A sound absorbent structure comprising a sheet having multiple holes therein, a layer of mineral wool disposed adjacent the sheet and extending substantially parallel thereto, a non-porous synthetic plastic tilm enclosing said layer, 'and a layer of solid foam disposed between said mineral wool layer and sheet, said foam layer having a thickness amounting to at least half the average diameter of the holes in said sheet.

5 4. A sound absorbent structure comprising a sheet having multiple holes therein, a layer of mineral wool disposed adjacent the sheet and extending substantially parallel thereto, a non-porous synthetic plastic film enclosing said layer, and an open grid-shaped spacing element disposed between said mineral wool layer and sheet, said element having a thickness amounting to at least half the average diameter of the .holes in said sheet.

References Cited by the Examiner UNITED STATES PATENTS 2,159,488 5/1939 Parkinson ISI-33.1

2,271,929 2/ 1942 Venzie 18 l-33.1

2,301,538 11/1942 Guyer et al. ISI- 33.1

7 0 FOREIGN PATENTS 1,138,543 1/1957 vFrance.

RICHARD B. WILKINSON, Primary Examiner.

R. S. WARD, JR., Assistant Examiner. 

1. A SOUND ABSORBENT CONSTRUCTION MATERIAL COMPRISING A FORAMINOUS SHEET, A LAYER OF FIBROUS SOUND ABSORBENT MATERIAL DISPOSED ADJACENT THE SHEET AND SUBSTANTIALLY PARALLEL THERETO, A SYNTHETIC FILM ENVELOPING SAID LAYER. AND MEANS TO SPACE SAID LAYER FROM THE SHEET BY A DISTANCE AT LEAST EQUAL TO ONE-HALF THE DIAMETER OF THE HOLES IN THE SHEET. 